1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium-sulfur battery comprising the same, and more particularly, to an organic electrolytic solution comprising a compound that can improve discharge capacity and cycle life of a lithium-sulfur battery, and a lithium-sulfur battery comprising the same.
2. Description of the Related Art
Demands for secondary batteries are increased according to rapid progress in portable electronic devices, and a battery having high energy density that can meet the present trend of light, thin, short and small dimensions in portable electronic devices is continuously required. A battery that can meet cheap, safe and environment-affinitive aspect is required to meet such need.
The lithium-sulfur battery of various batteries to meet such need is most promising in energy density among the batteries that have ever been developed. The energy density of lithium is 3830 mAh/g, and the energy density of sulfur (S8) is 1675 mAh/g. An active material used therein is cheap itself and environment-affinitive; however, such battery system has not been commercialized yet.
The reason why the lithium-sulfur battery cannot be commercialized is that the ratio of the amount of sulfur participating in electrochemical oxidation-reduction reaction within the battery to the amount of sulfur contained within the battery is so low that the battery shows low battery capacity.
For the lithium-sulfur battery, elemental sulfur is used as an initial anode active material. As the discharge of battery proceeds, eight sulfur atoms in cyclic molecular state are changed to linear molecular state while being reduced, and finally changed to S2− by continued reduction. The resulting S2− bonds chemically to surrounding lithium cations to form lithium sulfide (Li2S). Since the resulting lithium sulfide precipitates on an anode surface to reduce the activated area of the battery, and it cannot be oxidized during charging, the battery capacity is decreased. Accordingly, it is necessary to dissociate such lithium sulfide to maintain the activated area of the battery.
Approaches to solve such problems have been tried as follows.
U.S. Pat. No. 6,030,720 uses the solvent including R1(CH2CH2O)nR2 as a main solvent, in which n is 2 to 10, R1 and R2 are the same or different from each other, and represent a substituted or unsubstituted alkyl or alkoxy group, and a crown ether or a crypt and as a cosolvent. A donor or an acceptor cosolvent is included wherein the donor solvent has a donor number of at least 15. The separation distance of the battery must be 400 μm or less.
In general, when a lithium-sulfur battery is discharged, the formation and precipitation of Li2S on the surface of electrodes are known as a major cause of a drop in battery capacity. Many researches have been conducted to increase the capacity of the lithium-sulfur battery. In most cases, ether-based solvents capable of stabilizing the lithium-sulfides are used, and an initial discharge capacity of the lithium-sulfur battery is about 840 mAh/g-sulfur, which is about 50% of the theoretical capacity. The polar solvents such as DMF, DMAc, etc. were tried to dissociate Li2S, but the polar solvents vigorously react with lithium, and thus cannot be applied to the lithium-sulfur battery system.
Also, U.S. Pat. No. 5,961,672 suggests a mixed solution of 1M LiSO3CF3 and 1,3-dioxolane/diglyme/sulfolane/dimethoxyethane (50/20/10/20) as an electrolytic solution to improve lifetime and stability of a battery by coating a polymer film on a lithium metal anode.
U.S. Pat. No. 5,814,420 contacts the electrode containing active materials with both an ion conducting material and an electron conducting material such that the active material such as activated sulfur and/or polysulfide polymer can be almost completely used.
U.S. Pat. No. 5,523,179 discloses a lithium-sulfur battery comprising an active sulfur-based material, an ion conducting material and an electron conducting material in an anode.